OCL: GFTT small optimization

Removed 2 excess kernel call of `set`.
Thanks to @krodyush for pointing this.

**Performance report**
http://ocl.itseez.com/intel/export/perf/pr/3210/report/

check_regression=OCL*Good*
test_filter=OCL*Good*
build_examples=OFF
test_modules=imgproc
While sorting you read and write data. Then it has to be ACCESS_RW. Isn't it?